In general, although substances are expanded by heating, it has been known that the coefficient of linear expansion of an organic resin material is particularly high. For example, in a device represented by a device used for a precision optical system, in the case in which a member formed from an organic resin material is used, when the coefficient of linear expansion thereof is high, and the change in dimension of the member is large due to the change in temperature, the optical system may be disadvantageously displaced thereby. When a member used for a precision optical system is formed only from an organic resin material, the coefficient of linear expansion thereof is desired to be 20*10−6/degree Celsius or less.
In PTL 1 and PTL 2, as a method to prevent the displacement of an optical system caused by the thermal expansion of an organic resin material, a method has been disclosed in which a material having a negative linear expansion (hereinafter referred to as “negative expansion”) property is provided around a member formed of the organic resin material to compensate for the change in dimension.
In addition, in PTL 3 and PTL 4, as a method to prevent the displacement of an optical system caused by the thermal expansion of an organic resin material, a method has been disclosed in which inorganic fine particles are added to the organic resin material to decrease the coefficient of linear expansion.
In general, as a material having a negative expansion property, for example, inorganic materials, such as zirconium tungstate, a lithium-aluminum-silicon oxide, and a nitride of manganese, have been known.
However, in the already known method in which inorganic fine particles are added to an organic resin material to decrease the coefficient of linear expansion, a molded product having a coefficient of linear expansion of 10*10−6/degree Celsius or less is difficult to obtain, and hence the above method is difficult to be applied to a precision optical system and the like.
In addition, when the expansion of an organic resin material is compensated for by a negative expansion material of a manganese nitride, the absolute value of the coefficient of linear expansion of the inorganic material having a negative expansion property is small, such as 25*10−6/degree Celsius, at the maximum. As in the case described above, the absolute value of the coefficient of linear expansion of an organic material having a negative expansion property is also small, such as less than 10*10−6/degree Celsius. Therefore, in order to compensate for the expansion of an organic resin material caused by the change in temperature, a molded product formed using a material having a negative expansion property must have a considerable thickness or mass.